Measurement of Atomic and Molecular Excitation by a Trapped-Electron Method

A trapped-electron method is used to study inelastic processes in helium, mercury, and hydrogen. In this method, an electron beam traverses an electric and magnetic field configuration in which the low-energy electrons resulting from inelastic collisions are prevented from reaching the electron beam collector by a potential well. The low-energy electrons can reach a cylindrical collector surrounding the electron beam. The collection efficiency appears to be close to unity for electrons in the range 0 to 0.1 electron volt and zero at higher energies. The design of the tube and applied potentials determine the range of collection. A complete spectrum of energy levels is obtained in He, Hg, ${\mathrm{H}}_2$, within the resolution of the instrument (better than 0.2 ev). The experiment yields approximate slopes of the resolved excitation functions near threshold. The method is well suited for a study of excitation events in molecules.